1. Field of the Invention
The present invention relates to an apparatus for recording information signals on a recording medium and an apparatus for reproducing information signals recorded on the same.
2. Description of the Related Art
A magnetic type of recording and reproducing apparatus is known as one example of such an information signal recording and reproducing apparatus. Such a magnetic recording and reproducing apparatus typically employs so-called emphasis as a process for reducing noise components which may be introduced into a signal during its recording.
In general, when video signals are to be frequency-modulated and magnetically recorded, various adverse influences may occur such as moire due to side-band signals only if the deviation of frequency modulation is increased in order to reduce noise components. The above-described emphasis is a process which is widely used to eliminate such adverse influences. The emphasis process typically comprises the steps of increasing only the deviation of the high-band frequency components of an input video signal, recording the input video signal, and recovering the original level of the input video signal prior to reproducing it (that is, de-emphasis), whereby the influence of noise components during recording and reproduction is reduced.
FIG. 1(a) shows a typical example of the spectrum of noise components after frequency modulation and frequency demodulation. As is generally known, if the emphasis and de-emphasis processes are employed, such a spectrum is converted into a spectrum such as that shown in FIG. 1(b), so that the noise components can be reduced.
The above-described emphasis falls into two kinds of process. One process is called fixed emphasis and the other process is called dynamic emphasis These two processes will be described below with reference to a frequency modulation type of magnetic video-signal recording apparatus to which these processes are respectively applied.
FIG. 2 diagrammatically shows the construction of a magnetic recording apparatus of the type which uses the fixed emphasis. The illustrated magnetic recording apparatus includes a fixed emphasis circuit 200, a limiter circuit 201, a frequency modulation circuit 202, a recording amplifier 203 and a magnetic head 204 as well as a magnetic sheet 205 which serves as a magnetic recording medium. The fixed emphasis circuit 200 commonly has an amplification characteristic such as that shown in FIG. 3(a), and is constituted by a very simple circuit such as that shown in FIG. 3(b). Since the construction shown in FIG. 3(b) is of a general type, the description thereof is omitted.
Referring to FIG. 2, when the fixed emphasis circuit 200 receives an input video signal, the fixed emphasis circuit 200 amplifies the high-band frequency components of the input video signal on the basis of the amplification characteristic shown in FIG. 3(a) described above. As a result, sharp spike pulses called a white peak and a dark peak respectively occur in the steep rising and falling edges of a video signal, as shown in FIG. 2, and form a cause of a known inverted white peak or the like. For this reason, the limiter circuit 201 restricts the level of the video signal to a predetermined value to remove the spike pulses, and the signal thus shaped is frequency-modulated by the frequency modulation circuit 202. In consequence, the frequency-modulated signal output from the frequency modulation circuit 202 is a signal obtained by frequency-modulating the signal whose high-band frequency components are amplified by the fixed emphasis circuit 200. It follows, therefore, that frequency modulation has been effected such that the deviation of the high-band frequency components is large compared with that of the low-band frequency components.
The signal thus frequency-modulated is amplified by the recording amplifier 203 and then recorded through the magnetic head 204 on the magnetic sheet 205 which is rotated by a motor (not shown).
FIG. 4 diagrammatically shows the construction of a magnetic recording apparatus of the type which employs the dynamic emphasis. The illustrated apparatus includes a dynamic emphasis circuit 400, a limiter circuit 401, a frequency modulation circuit 402, a recording amplifier 403 and a magnetic head 404 as well as a magnetic sheet 405 which serves as the magnetic recording medium. The dynamic emphasis circuit 400 has a characteristic in which its amplification ratio non-linearly varies in accordance with the level of an input signal. Accordingly, the dynamic emphasis circuit 400 is capable of achieving a noise reduction effect higher than the fixed emphasis circuit 200. FIG. 5(a) is a graphic representation showing the amplification characteristic of the dynamic emphasis circuit 400, and FIG. 5(b) diagrammatically shows a concrete example of the construction of the same. Since the construction shown in FIG. 5(b) is of a general type, the description thereof is omitted.
Referring to FIG. 4, when the dynamic emphasis circuit 400 receives an input video signal, the dynamic emphasis circuit 400 amplifies the high-band frequency components of the input video signal on the basis of the amplification characteristic shown in FIG. 5(a) described above. As a result, a white peak and a dark peak respectively occur in the rising and falling edges of the video signal, as in the case of the fixed emphasis circuit 200 described above, but as the level of the input video signal is lower, the high-band frequency components are amplified at a higher amplification ratio. Accordingly, adaptive amplification is performed in accordance with the level of each input video signal.
The video signal which has thus been subjected to the dynamic emphasis is restricted in amplitude by the limiter circuit 401, and thus the white and dark peaks are removed. The signal thus shaped is frequency-modulated by the frequency modulation circuit 402 and then recorded through the magnetic head 404 on the magnetic sheet 405 which is rotated by a motor (not shown).
As described above, when a video signal is recorded by a recording apparatus having the above-described emphasis circuit and is recovered by a reproducing apparatus through the de-emphasis circuit thereof, the noise components contained in the high-band frequency components of the obtained signal are reduced as shown in FIG. 1(b). Such a signal can be easily reproduced by a reproduction apparatus such as that shown in FIG. 6.
The reproducing apparatus shown in FIG. 6 includes, in addition to a magnetic sheet 600 which serves as a magnetic recording medium, a magnetic head 601, a pre-amplifier 602, a frequency demodulation circuit 603 and a de-emphasis circuit 604. The signal reproduced from the magnetic sheet 600 by the magnetic head 601 is amplified by the pre-amplifier 602, demodulated by the frequency demodulation circuit 603, and applied to the de-emphasis circuit 604 which has the characteristics reverse to those of the fixed emphasis circuit 200 and the dynamic emphasis circuit 604. Thus, the original video signal is recovered and output.
When the noise components of a signal are to be reduced by using each of the above-described emphasis circuits, the deviation of the high-band frequency components of the signal increases during frequency modulation as the amplification ratio of the high-band frequency components is increased. As a result, it is possible to obtain a large reduction effect for the noise components.
However, if the amplification ratio of the high-band frequency components is made excessively high, a known inverted white peak occurs during frequency demodulation in a reproduction process, and it may become impossible to recover the signal correctly.
The inverted white peak is a phenomenon in which, if a signal containing a sharp peak generated by an emphasis circuit is recorded after frequency modulation, a zero crossing point may be lost in a spectral portion of a reproduced signal that corresponds to the sharp peak, with the result that, if the recorded signal is a video signal, a signal which originally represents white is recovered as a signal which represents black, or vice versa. To prevent the occurrence of such an inverted white peak, signals, after the above-described emphasis, are limited in amplitude by a limiter circuit to suppress the level of the aforesaid peak to some degree.
However, if signals are excessively limited, a phenomenon known as smear occurs and causes reproduced signals to deteriorate. For this reason, the fixed emphasis is limited in increasing the amplification ratio.
The dynamic emphasis has the problem that, since its non-linear characteristic cannot be exactly set when the interchangeability between devices is taken into account, it is impossible to increase the amplification ratio beyond a certain degree.
In particular, there has recently been a strong demand for high-quality recording and reproduction of video signals. In response to this demand, so-called high-band recording is utilized for recording video signals after frequency modulation using a carrier signal of high frequency. In such high-band recording, it is necessary to increase the amplification ratio in emphasis in order to reduce noise components. In a conventional emphasis process, however, since it is difficult to increase the amplification ratio in emphasis as described previously, it has been impossible to achieve satisfactory reduction of noise components.
Also, if a disc-shaped magnetic sheet is used as the recording medium, the magnetic sheet is rotated at a predetermined rotational speed, and frequency-modulated video signals are recorded on the rotating magnetic sheet. In this case, a plurality of recording tracks are concentrically formed on the magnetic sheet, and the relative speed of the magnetic sheet with respect to the magnetic head on the recording tracks in the outer circumferential portion differs from that on the recording tracks in the inner circumferential portion. As a result, the shortest recording wavelengths grow shorter from the outer circumference to the inner circumference. For this reason, if the S/N (signal-to-noise) ratio is to be improved by emphasis as described above when the aforesaid video signals are recorded after an emphasis process such as that described above, the inverted white peak occurs on the inner circumferential portion if emphasis is effected on signals to be recorded on the outer circumferential portion to such an extent that the inverted white peak does not occur.
On the other hand, if emphasis is effected on signals to be recorded on the inner circumferential portion to such an extent that the inverted white peak does not occur, no inverted white peak occurs in the outer circumferential portion, but no great improvement in S/N ratio is obtained. In a conventional type of emphasis process, emphasis can only be effected to such an extent that no inverted white peak occurs on the inner or outer circumferential portion. Therefore, a recording operation which is disadvantageous in terms of the S/N ratio must be performed.
Also, VTRs (video tape recorders) or VD systems (video disc systems) for home use, if visually tolerable, need not necessarily record or reproduce television signals with fidelity as high as that of the broadcasting VTRs. With this viewpoint, the following noise reduction systems are commonly adopted in VTRs or VD systems for home use. Such noise reduction systems fall into two major types. One type is called an addition type and the other a subtraction type. In the addition type of noise reduction system, a reproduced luminance signal is divided into a high-band signal and a low-band signal by means of a filter. In general, noise which is relatively conspicuous appears as a flat object reproduced from an input image, for example, a large-area flat object such as the sky or a wall, and there is a tendency for such noise to gather about a low-level portion of the high-band signal. In the addition type of noise reduction system, such noise is removed by a slicer circuit, and the output of the slicer circuit, after level matching, is superimposed on the low-band signal In the subtraction type of noise reduction system, noise is extracted from the high-band signal by means of a limiter, and the noise is reversed in phase and superimposed on an input luminance signal, thereby cancelling the noise.
In addition, a noise reduction circuit called the "DDC (double differential circuit)" has been proposed as a modified version of the former addition type noise reduction circuit. The DDC is designed to increase the level of a high-band signal to some extent by means of a quadratic differential type phase compensating circuit, to add such a high-band signal to a low-band signal, thereby increasing the sharpness of the contour of an input image.
Furthermore, a horizontal-correlation noise reduction circuit employing a one-horizontal-scanning-period delay circuit (or 1-H delay line) is proposed as a high-performance version of such a noise reduction system. The horizontal-correlation noise reduction circuit makes use of the fact that, since a high line correlation (high vertical correlation) is established between a reproduced luminance signal and a 1-H delayed signal, noise components without line correlation are obtained as the difference output therebetween. The residual noise components which have thus been extracted are passed through a limiter and then superimposed on the original signal (as disclosed in, for example, Japanese Patent Application Laid-open No. Sho 60-30285). A further improved version (as disclosed in, for example, Japanese Patent Application Laid-open No. Sho 60-121885) is also proposed or actually utilized.
In all of the above-described conventional examples, however, whichever noise reduction system may be employed, noise reduction is achieved at the expense of a decrease in the high-band frequency components, an increase in waveform distortion, and an increase in waveform distortion in a portion without vertical correlation. In particular, as the amount of noise reduction is increased, these disadvantages grow large.
Furthermore, a system in which a disc-shaped recording medium (a magnetic or optical type of disc) is rotated at a constant angular velocity and video signals which are frequency-modulated with a fixed frequency are recorded on such a recording medium encounters the following problem. That is, since the recording wavelengths grow shorter as the recording proceeds toward the inner circumference of the recording medium, the S/N ratio deteriorates.
The above-described problems will be described in greater detail below with reference to FIGS. 7 and 8. FIG. 7 diagrammatically shows a typical basic circuit for effecting noise reduction and FIG. 8 shows in graphic form the waveform provided at each portion of the circuit shown in FIG. 7.
First, a signal of a waveform such as that shown in part S2 of FIG. 8 is input to a low-pass filter (LPF) 700 and a high-pass filter (HPF) 701 which are shown in FIG. 7. One signal is filtered by the low-pass filter 700 to be transformed into a low-band signal of a waveform such as that shown in part S2 of FIG. 8, and then applied to an adder 702. The other signal is filtered by the high-pass filter 701 to be transformed into a high-band signal of a waveform such as that shown in part S3 of FIG. 8, and then applied to a limiter 703. The limiter 703 slices a low-level portion of the high-band signal in the range represented as D in part S3 of FIG. 8 to provide a signal of a waveform such as that shown in part S4(a) of FIG. 8. When this signal is added to the signal S2 in the adder 702, a signal of a waveform such as that shown in part S5(a) of FIG. 8 is obtained. As can be seen from the waveform S5(a), the noise is greatly reduced but the high-frequency portions of the rising and falling edges (contour portion) of the signal are remarkably rounded. As a result, a picture devoid of sharpness is reproduced.
If the slicing level of the limiter 703 is further decreased, the limiter 703 outputs a waveform such as that shown in part S4(b) of FIG. 8, and its final output waveform becomes a waveform such as that shown in part S5(b) of FIG. 8. In this case, the degree of noise reduction is small, but the sharpness of the reproduced image is relatively great since its waveform distortion is small.
Various improvements have recently been proposed with respect to the noise reduction circuit described above with reference to the most basic example thereof, and the effect of noise reduction has been significantly improved. However, there is still a limitation in that, if the amount of noise reduction is increased, a significant waveform distortion occurs as shown in part S5(a) of FIG. 8 and in that, if the waveform distortion is suppressed, the amount of noise reduction decreases as shown in part S5(b) of FIG. 8.
As is known, still video systems for recording and reproducing still pictures are arranged such that still picture signals are frequency-modulated and concentrically recorded on a 2-in. dia. magnetic disc of up to fifty fields. The luminance signal of each picture signal is frequency-modulated in the range of from 6 MHz to 7.5 MHz, which is equivalent to the range from sync chip to white peak. The chrominance signal of the same, after a color-difference line-sequential process, is frequency-modulated by using about 1 MHz as the center frequency and then recorded on the above-described disc. From this, it follows that signals of substantially the same spectrum are recorded concentrically from the outer circumference to the inner circumference. As a result, recording wavelengths grow shorter from the outer circumference toward the inner circumference.
As is well known, as the recording waveform grows short, the C/N ratio (carrier-to-noise ratio) of an electromagnetic system deteriorates. Hence, the S/N ratio of the picture signals which have been thus recorded and reproduced deteriorates. For this reason, although a certain degree of image quality can be achieved around the outermost circumference of a magnetic disc, the image quality progressively deteriorates as recording proceeds from the outer circumference to the inner circumference. Around the innermost circumference, luminance signals distinctly deteriorate in S/N ratio. If an enhanced carrier frequency is used in frequency modulation to allow for high-definition reproduction, the S/N ratio deteriorates to a remarkable extent.